Identification of candidate CSF metabolic biomarkers in CNS lymphoma. (A) Relative quantification of metabolites in CSF in 14 control subjects without brain tumors and 14 independent patients with CNS lymphoma are presented in the heat map. Of 145 metabolites analyzed, 36 were significantly upregulated in CNS lymphoma (P < .05, Wilcoxon test). Notably, among the upregulated metabolites in CSF in CNS lymphoma, lactate and kynurenine were most significantly increased in patients with active leptomeningeal lymphoma. In addition, CSF from CNS lymphoma patients contained an increased ratio of kynurenine/tryptophan, indicative of activation of indoleamine-2,3 dioxygenase in CNS lymphoma (fold-change 2.3, P < .039). Using a standardized quantitative assay for CSF lactate (Beckman Coulter Unicell Dxc 800 Clinical Chemistry Analyzer), we confirmed upregulated CSF lactate by more than twofold in an independent validation set of 8 CNS DLBCL vs 8 nonneoplastic controls (P < .0015) (supplemental Figure 2). We also determined that elevated baseline CSF lactate (>ULN 2.8 mmol/L), correlated with short PFS (P = .0129) as well as OS (P = .004) in relapsed CNS lymphoma patients treated with lenalidomide monotherapy. We determined lactate concentrations in ventricular CSF in pretreatment specimens in 33 patients with relapsed CNS lymphoma that participated in 3 independent phase 1 immunotherapy trials involving rituximab (8/10 pretreatment specimens),²⁴  methotrexate plus rituximab (11/14 pretreatment specimens),²⁵  and lenalidomide plus rituximab (all 14 pretreatment specimens). Median baseline lactate concentration in ventricular CSF in these patients was 2.4 mmol/L (range, 1.2-9). Patients with elevated lactate in ventricular CSF (>2.8 mmol/L, ULN of CSF lactate) experienced shorter overall survival in each of these individual trials compared with patients with normal lactate at baseline. In aggregate, patients with elevated CSF lactate (12 patients) exhibited significantly shorter OS compared with patients with low CSF lactate (22 patients) P < .0001. (B) High CSF lactate also correlated with shorter progression-free survival in relapsed CNS lymphoma subjects treated with lenalidomide monotherapy. (C) By contrast, serum LDH at baseline did not significantly correlate with PFS or OS (not shown). Notably, pretreatment baseline, matched CSF, and plasma lactate concentrations were compared in 7 lenalidomide/rituximab trial subjects: in 5, the concentration of lactate in CSF was higher (median, 1.9-fold higher) than the concentration of lactate in plasma.